The present invention relates to an improvement in the method of starting a variable-speed pump turbine of a pumped storage power station and, in particular, to an improvement in the method of starting a variable-speed pump turbine which is operated at varying speeds as a wound-rotor induction motor.
The current increase in nuclear power plants has given rise to demands for the automatic frequency control of hydraulic power stations to increase the proportion of partial-load operation in these hydraulic power stations. In consequence, there is a need to improve the efficiency of a hydraulic power station during partial-load operation.
In recent years hydraulic power stations have often been built at such places at which differences in the water head are large, due to location restrictions. This in turn requires a suitable countermeasure to prevent any drop in efficiency due to changes in the head.
Some ideas for preventing a drop of efficiency due to changes in the head have been proposed, of these the most practical suggestion is to permit the power generation system to work at a variable speed to improve its efficiency.
In such a power generating system, the efficiency of the water turbine which drives the generator is reduced undesirably when the head decreases, but any drop in efficiency can be suppressed by varying the speed of the water turbine according to the changes in the head. The variable-speed power generation system offers another advantage in that, for a pumped storage power station, the pumping rate can be adjusted to make efficient use of surplus power. Namely, a pumped storage power generation system designed for constant-speed operation requires a constant pump rate or constant electric power and, hence, cannot pump water at a rate matched to surplus power. Adjustment of the operation of a constant-speed pumped storage power plant to obtain a pump rate matched to surplus power would require an impracticably large quantity of equipment for the adjustment.
If the alternator motor is a synchronous motor, a variable-speed operation can be achieved by connecting a thyristor frequency converter between the alternator motor and the power line, and varying the frequency of the converter. In this case, however, the thyristor converter must have a large capacity matched to the maximum level of power generation, so that the production cost and power losses during operation increase.
One solution to this problem is to use a secondary-excitation induction motor as the motor generator. In this case, the secondary-excitation controller is designed to provide a narrow speed control range around the synchronous speed of the induction motor. With this arrangement, it is possible to reduce the capacity of the thyristor converter. There are two systems for connecting the thyristor to the secondary winding of the induction motor to effect secondary excitation speed control: namely a system called the separate-excitation inverter system in which the A.C. current is converted into D.C. current and then back to an A.C. current of a frequency coinciding with the slip frequency of the induction motor; and a cycloconverter system in which A.C. current is produced directly from the A.C. current available. The separate-excitation inverter system, however, has the problem that the primary current waveform sent to the power line has several higher harmonics because the output from the inverter, i.e. the secondary current in the induction motor, has a rectangular waveform. This has an unfavourable effect on the power line, particularly when large-capacity machinery is used. In addition, since a lagging power factor is not possible, the excitation current is supplied from an external power line so that lagging reactive power is consumed, which seriously affects the line voltage. On the other hand, the cycloconverter system produces few higher harmonics because it is possible to use a substantially sine-wave secondary current for the induction motor. In addition, this system enables a control of the line voltage by variation of the power factor of the output to the power line, because it is possible to control the power factor of the output of a cycloconverter.
The cycloconverter system, although it has the above advantages, cannot be applied directly to the start-up of a pump turbine, because its speed can only be controlled within a small range. Namely, in order that this system can be used satisfactorily in the start-up of a pump turbine, it is essential that the output frequency of the thyristor converter can be varied over a wide range between zero and the rated power source frequency. More specifically, in a large-capacity induction motor, the torque increases dramatically in the region near zero slip while, in the region of large slip, the motor does not produce substantial torque. When starting up an induction motor of a pumped storage power station, it is necessary to vary the source frequency of the induction motor so that it always operates in the region near zero slip.
To realize such a control, it is possible to operate the thyristor converter of the cycloconverter as a AC-DC-AC converter only during the start-up of the induction motor. To this end, however, it is necessary to employ a large number of change-over breakers for switching the circuits of the converter. This in turn requires a large installation space within the power plant for the machinery, as well as a complicated sequence for the switching of the circuit, resulting in increased production and installation costs.